The invention relates to the fabrication of spherical members such as inertially imploded targets, and more particularly to apparatus and method utilizing an electrical heat pulse for producing such targets having at least one uniform layer of cryogenic material therein.
Various types of targets have been proposed, both single and multiple shell, for use in inertial confinement systems, wherein the targets are imploded by energy from lasers, electron-beams, and ion-beams. The targets which have been experimentally tested have been primarily those composed of hollow glass microspheres filled with fuel such as DT in the gaseous, liquid or solid form. While some experiments have involved targets wherein the DT is formed as a hollow shell within the glass microsphere, one of the problems of this type of target is that the shell or layer of fuel is not of uniform thickness, thereby producing results that are not as satisfactory as desired. These hollow fuel shell targets were produced by diffusion of the DT through the glass microspheres and then reducing the temperature so as to "freeze" the DT on the inner surface of the glass microspheres forming a DT shell therein. Tests have shown that while this technique resulted in producing a shell of DT within the glass microsphere, the thickness of the shell was sometimes grossly nonuniform, and efforts have been directed to developing methods for forming a more uniform shell or deposit of DT. One prior approach to this problem involved conduction cooling of the target through the target support wherein the target was secured to a cooled, thin metal fiber that provided the refrigeration whereby DT's heat of vaporization is removed causing "freezing" of the DT within the glass shell. While this approach provided improved results, the targets thus made tended to have a thicker DT layer in the area of the cooled support, and thus were not sufficiently uniform. Another approach to this problem involved heat removal through an exchange gas surrounding the glass microsphere and wherein the target was slowly spun in quasi-isothermal conditions, but the results again were found to not be as satisfactory as desired for producing a uniform DT layer or shell within the glass microsphere. Thus a need has existed in the prior art for an effective method and/or apparatus for producing uniform DT, or other cryogenic material, layers or shells on the inner surface of a containing hollow member, such as the glass microspheres used in inertial confinement targets.
Targets of this type have many applications as set forth in U.S. Pat. No. 3,723,246 to M. J. Lubin, such as providing high temperature plasma for use in linear and/or closed plasma research apparatus, such as stellarators, tokamaks, magnetic mirrors, etc. In addition, plasma produced by inertially imploded targets can be used for providing space propulsion, neutron production, physics studies, etc.
It has recently been found by work carried out at the Los Alamos Scientific Laboratory, for example, that by vaporizing and rapidly cooling cryogenic materials, such as DT, contained in a hollow spherical member, such as a glass microsphere, located within an isothermal freezing cell, a uniform layer of the cryogenic material will be formed inside the hollow spherical member. This can be accomplished, for example by the so-called light pulse method described and claimed in U.S. patent application No. Ser. No. 872,284, filed Jan. 25, 1978, in the name of J. R. Miller, assigned to the assignee of this application and also described in a paper (LA-UR-76-2353) by J. R. Miller entitled, "A New Method For Producing Cryogenic Laser-Fusion Targets", presented at the 18th Annual Meeting of the Plasma Physics Division of the American Physical Society, San Francisco, California, Nov. 15-19, 1976, wherein a target composed of a glass microballoon filled with DT is positioned in a multiple shield isothermal freezing cell and is acted on by a light pulse, such as a laser heat source, wherein sufficient energy is absorbed by the glass microballoon to vaporize the DT, and upon removal of the laser heat source the DT is quickly frozen forming a uniform layer or shell within the glass microballoon.